1. Field of the Invention
The present invention relates to a device (hereinafter referred to as “light-emitting device”) having an element (hereinafter referred to as “light-emitting element”) in which a thin film made of a light-emissive material is interposed between a pair of electrodes (an anode and a cathode). In particular, the invention relates to a light-emitting device having a light-emitting element (hereinafter referred to as “EL element”) using a thin film that is made of a light-emissive material capable of EL (electroluminescence). The organic EL display and the organic light-emitting diode (OLED) are included in the light-emitting device according to the invention.
All light-emissive materials that emit light (phosphorescence and/or fluorescence) through singlet excitation or triplet excitation or both are part of the light-emissive materials that can be used in the invention.
2. Description of the Related Art
Light-emitting devices (hereinafter referred to as “EL light-emitting devices”) having an EL element are now being developed. EL light-emitting devices are classified into the passive matrix type and the active matrix type, and EL light-emitting devices of both types operate on the principle that a current is caused to flow through an EL element and thereby a thin film (light-emitting layer) made of a light-emissive material capable of EL emits light.
FIG. 2 shows the structure of a general EL element. As shown in FIG. 2, an EL element 200 is formed by laying an anode 202, a light-emitting layer 203, and a cathode 204 one on another on an insulator 201. In general, the cathode 204 as an electron supply source is a metal electrode having a small work function and the anode 202 as a hole supply source is an oxide conductive film (typically, an ITO film) that has a large work function and is transparent to visible light. This is because the metal electrode as the cathode 204 is opaque to visible light and hence light (hereinafter referred to as EL light) that is generated in the light-emitting layer cannot be observed unless the anode 202 is transparent to visible light.
The EL light 205 is observed after directly passing through the anode 202 or being reflected by the cathode 204 and then passing through the anode 202. That is, an observer 206 can observe the EL light 205 that originates from pixels where the light-emitting layer 203 emits light and that then passes through the anode 202.
However, there is a problem that incident ambient light (i.e., light outside the light-emitting device) 207 is reflected by the back surface (i.e., the surface adjacent to the light emitting layer) of the cathode 204 in pixels that are not emitting light and the back surface of the cathode 204 acts like a mirror, whereby an external view appears on the observation surface (i.e., the surface on the observer 206 side). One measure against this problem is to attach a circularly polarizing film to the observation surface of an EL light-emitting device. However, a problem still exists that the circularly polarizing film is very expensive and hence its employment increases the manufacturing cost.